Venus’ transit day has passed – the last transit of Venus for the 21st century! The brightest planet, Venus, passed right in front of the sun for nearly seven hours on June 5-6, 2012, but, from many places, the transit was in progress at sunrise or sunset. During the transit, Venus appeared in silhouette as a small, dark dot moving in front of the solar disk. This exceedingly rare astronomical event – a transit of Venus – won’t happen again until December 11, 2117. However, a transit of Mercury will take place on May 9, 2016.

As with any solar eclipse, you must have proper eye protection to view a planet transiting across the sun. Click on the following link to hear why eclipses glasses and welder’s glass might not be best for viewing a solar eclipse, to learn how to make an indirect viewing system, and to find a webcast: What’s the best way to view the June 5-6 transit of Venus safely?

Transit of Venus June 8, 2004. Image Credit: Wikimedia Commons

Who saw the June 5-6, 2012 transit of Venus?

Depending on where you live worldwide, the transit of Venus happened on June 5 or 6, 2012. If you live in the world’s Western Hemisphere (North America, northwestern South America, Hawaii, Greenland or Iceland), the transit started in the afternoon hours on June 5. In the world’s Eastern Hemisphere (Europe, Africa, Asia, Australia or New Zealand), the transit was first seen at sunrise or in the morning hours on June 6.

Map of where June 5-6, 2012 Venus transit will be seen. Image Credit: Michael Zeiler, eclipse-maps.com

The chart above shows the times of the 2012 transit of Venus in Universal Time (essentially the same as UTC). To know when any astronomical event occurs in your part of the world, you need to translate the times to your time zone. Here’s how to do that: How do I translate Universal Time into my time?

Be careful and watch for the time of sunset or sunrise in your location. In the continental United States, for example, the greatest transit took place just before sunset for most of us. During last month’s solar eclipse, which was also at sunset for U.S. locations, many people told us they missed out because their viewing location was hindered by trees or tall buildings. Don’t let that happen to you! If you’re in the continental U.S., be sure to find a viewing location with a clear view of the horizon. To know the time of sunset or sunrise for your location, try this site, which will let you create a custom sunrise/sunset calendar: Sunrise/sunset times for your sky

From the mainland U.S., the West Coast saw more hours of the transit than the East Coast did. Here in Austin, Texas, we got to see the first half of the transit, but the second half took place after sunset – or when the sun was beneath our horizon.

In the world’s Eastern Hemisphere, it was as equally important to find a level eastern horizon on June 6. For Africa, Europe, much of Asia and western Australia, the sun rose (on June 6) as the transit was taking place. Elsewhere in the Eastern Hemisphere, the transit started in the morning hours on June 6.

The contact times (I, II, greatest transit, III and IV) on the above illustration are given in Universal Time. It is important to note that these times are for an imaginary observer at the Earth’s center – not the Earth’s surface. So if you translate from Universal Time to the clock time in your time zone, it’ll give you a ballpark reference of your local transit times, which won’t be off by any more than a maximum of plus or minus seven minutes.

Why is a transit of Venus so rare?

The last transit of Venus was June 8, 2004. But don’t be fooled by that proximity in time. Transits of Venus are very rare, plus transits tend to occur in pairs. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of up to 121.5 years. Before 2004, the last pair of transits were in December 1874 and December 1882.

Venus, the second planet outward from the sun and next planet inward from Earth, swings between the Earth and sun (at a point called inferior conjunction) five times every eight years, or one time in every 584 or so days. (See the Diagram of Venus’ orbit around the sun below.) More often than not, Venus passes above or below the solar disk at inferior conjunction – that point in its orbit where Venus passes out Earth’s evening sky and into Earth’s morning sky.

Diagram of Venus’ orbit around the sun

Venus passes in between the Earth and sun at inferior conjunction

Top: Venus at inferior conjunction not aligned with node; Bottom: Venus at inferior conjunction aligned with node. At inferior conjunction, Venus passes between the Earth and sun.

If Venus and Earth revolved around the sun on the same plane, there would be five inferior conjunctions – and five transits – of Venus every eight years. However, Venus’ orbital plane is inclined to Earth’s orbital plane by 3.4o. Because the orbital planes of the two planets don’t quite mesh, a combination of factors is necessary for a transit of Venus to take place in Earth’s sky.

For half of Venus’ orbit, Venus travels south of the Earth’s orbital plane, and for the other half of Venus’ orbit, Venus travels north of the Earth’s orbital plane. At two places in Venus’ orbit, Venus crosses the Earth’s orbital plane at points called nodes. If Venus is going from south to north, it’s called an ascending node, or if going from north to south, it’s called a descending node.

If Venus at inferior conjunction closely coincides with one of its nodes, then a transit of Venus is in the works. On June 5-6, 2012, Venus swings to inferior conjunction and sufficiently close to its descending node to present the last transit of Venus until December 11, 2117.

Bottom line: Here’s everything you need to know about the last transit of Venus in this century on June 5-6, 2012. The exact date will depend on your hemisphere on Earth. During the transit, Venus will appear in silhouette as a small, dark dot moving in front of the solar disk. The next transit of Venus won’t be until December 11, 2117. This post has times of the transit, and links to tips for safe viewing and more.